I'm sure it would not be too hard to move the AN forum to another platform (such as the Simple Machines platform used here on UKC) however I suspect that it is the huge database that is the difficult part and ultimately, that is what makes AN what it is. There is such a vast wealth of information stored there it would be a great shame to lose it, or at least, lose access to it, even if the data is safe.
The AN website is / was built as a whole: while the forum and mines database are sort-of separate entities, they're very closely linked eg so that images can be uploaded, hosted and easily put into forum posts); not to mention the code for things like the events system, the 'I worked here' feature, photo collections, photo comments, the ability to add locations etc. - all of which used the forum login for access.
It would have been a lot easier to have an off-the-shelf forum but it wouldn't have integrated so well - and a lot of what the site did might not have been possible.
The problem with this was that some of original code has got to the stage where modern servers can't host it and a lot of re-writing is necessary, which requires a huge amount of time - and that is a rare commodity.
With respect to the many kind offers of help, because the whole platform is bespoke it's unlikely that anyone other than the person who originally built the site could come in and work on it.
What will hopefully come back is a new forum (either bespoke or off-the-shelf) and an updated database. Much of the existing coding for the mines database is more modern, but the admin side requires work.
Some things won't return, but arguably the world has moved on in 10 years or so and some aspects are no longer required / no longer justified (and in any case were rarely used). Users won't notice the difference 99% of the time.
As an admin of AN but also from a personal point of view, I would just like to take this opportunity to thank SimonRL for all his work in creating the site and maintaining it over 15 years (so far) but also to the many, many people who have assisted with managing the website and - more importantly - who have contributed data; added obscure location; edited the mines database; added descriptions and grid references to locations; corrected things; added documents and plans; added photographs; went out and confirmed things in the field; got into long-lost places and recorded them; and generally tweaked and polished things to make the AN database what it now is. Most of this has been done without fanfare and without public comment but it has been massively appreciated: thank you!